Commercial radiation curable inkjet inks contain significant amounts of low molecular weight photo-initiators. Especially Norrish type II-initiators such as isopropyl thioxanthone or benzophenone derivatives are prone to diffuse out of the curable composition even after curing, as they are not built into the polymer network. This tendency to migrate and the possibility of being extracted into the food makes them less suitable for application in inks for food packaging. Norrish type I initiators are normally built into the network, except when the quantum efficiency for cleavage is less than unity or when side reactions occur yielding extractable degradation products. Unreacted initiators may also deteriorate the physical properties of the packaging material.
One approach to solve these problems is to design initiators with a higher molecular weight.
Polymeric initiators have been disclosed in CRIVELLO, J. V., et al. Photoinitiators for Free Radical Cationic and Anionic Photopolymerisation. Surface Coatings Technology. 1998, vol. III, p. 208-224. and CORRALES, T., et al. Free radical macrophotoinitiators: an overview on recent advances. Journal of Photochemistry and Photobiology A: Chemistry. 2003, vol. 159, no. 2, p. 103-114. All the disclosed polymeric initiators have a conventional linear molecular geometry. The solution viscosity of a radiation curable composition is influenced significantly using these polymeric initiators. Especially in ink jet applications, a significant increase in the solution viscosity has to be avoided to keep the inkjet ink jettable.
Some multifunctional initiators for food applications have been disclosed. WO 03033452 (COATES BROTHERS PLC) discloses multifunctional benzophenone initiators having the following general structure:
where n is a number from 1 to 6; R3 is hydrogen, methyl or ethyl; A represents a group of formula —[O(CHR2CHR1)a]y—, —[O(CH2)bCO]y, or —[O(CH2CO](y-1)—[O(CHR2CHR1)a]— (where one of R1 and R2 is hydrogen and the other is hydrogen, methyl or ethyl); a is from 1 to 2; b is from 4 to 5; y is from 3 to 10; Q is a residue of a polyhydroxy compound having 2 to 6 hydroxyl groups; and x is greater than 1 but no greater than the number of available hydroxyl groups in Q.
WO 03033492 (COATES BROTHERS PLC) discloses similar polymeric initiators having the following structure:
where n is a number from 1 to 6; R3 is hydrogen, methyl or ethyl; A represents a group of formula —[O(CHR2CHR1)a]y—, —[O(CH2)bCO]y, or —[O(CH2CO](y-1)—[O(CHR2CHR1)a]— (where one of R1 and R2 is hydrogen and the other is hydrogen, methyl or ethyl); a is from 1 to 2; b is from 4 to 5; y is from 3 to 10; Q is a residue of a polyhydroxy compound having 2 to 6 hydroxyl groups; and x is greater than 1 but no greater than the number of available hydroxyl groups in Q.
Both WO 03033452 (COATES BROTHERS PLC) and WO 03033492 (COATES BROTHERS PLC) teach that the molecular weight of the multifunctional initiators is most preferably lower than 800, since higher molecular weights cause an unwanted increase in the viscosity of the radiation curable formulation. This limits the functionality of the multifunctional initiator and limits the possibilities to optimize physical properties, such as the compatibility with different radiation curable compositions, to the choice of Q. Using part of the hydroxyl groups of the core to introduce moieties for optimization of physical properties would lead to multifunctional initiators with a low functionality. High concentrations of photoinitiators would then be needed to obtain the required curing sensitivity, thus limiting the possibilities for the composition and having a large influence on the properties of the composition and the final result.
WO 9717378 (COATES BROTHERS PLC) discloses a different type of multifunctional initiators obtained by the reaction of a multifunctional core material containing two or more reactive groups and a photoinitiator or derivative thereof. The photoinitiator or derivative thereof has a reactive group capable of reacting with the reactive groups of the multifunctional core. The photoinitiators disclosed in WO 9717378 (COATES BROTHERS PLC) are low molecular weight compounds, having a maximum functionality of 6. Depending on the functionality of the polyfunctional initiator, the molecular weight of the core is preferably less than 500 for a difunctional initiator, preferable less than 1000 for a tetrafunctional initiator and less than 1500 for a hexafunctional initiator. For inkjet applications, a further increase of molecular weight would lead to an unacceptable viscosity of the radiation curable inkjet ink.
WO 9749664 (LAMBSON FINE CHEMICALS) discloses a photoinitiator, comprising a photoreactive portion and a pendant group, the photoreactive portion including an aromatic moiety and the pending group incorporating at least one optionally substituted poly(alkylene glycol) moiety. Preferred photoreactive portions include optionally substituted benzophenone, thioxanthone and anthraquinone compounds substituted by a polyethylene glycol or polypropylene glycol moiety of an average molecular weight in the range 150 to 900. These types of initiators are essentially monofunctional. The molecular weight per photoreactive moiety is high. For an equal molar initiator concentration, a high weight percentage of these macromolecular initiators are required compared to their low molecular weight counterparts. As a result a high amount of unreactive polymer is introduced in the radiation curable formulation, having a negative influence on physical properties such as scratch resistance.
There is therefore a need to provide radiation curable compositions with an initiator that combines a high functionality with a limited influence on viscosity. These radiation curable compositions should be suitable for use on food packaging materials, whereby the initiators can not be extracted into the food or deteriorate the physical properties of the packaging material. The initiator should further be easy to manufacture and compatible with many different radiation curable compositions.